Method of manufacturing semiconductor device

ABSTRACT

On a substrate ( 1 ), a silicon oxide film ( 2 ) is formed. Thereon, a silicon nitride film ( 3 ) is formed. And, an opening for a trench is formed by patterning these films. Then, an oxide film including fluorine ( 8 ) is formed on the substrate ( 1 ). And, a fluorine diffusing layer ( 9 ) is formed by diffusing fluorine from the oxide film including fluorine ( 8 ) with its width wider than the opening for a trench. Then, a trench ( 4 ) is formed, remaining fluorine diffusing layer portions ( 9   a ). After this, a thermal oxide film is formed on the inner wall of trench ( 4 ). Then, the thermal oxide film includes fluorine near the corner of trench ( 4 ). This is diffused from the fluorine diffusing layer portions ( 9   a ). After all, the trench ( 4 ) is filled with oxide film ( 6 ).

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention relates to a method of manufacturing semiconductor device for forming isolation regions of semiconductor elements, which is formed on a semiconductor substrate, in order to isolate elements by insulator. And, it especially relates to a method to effectively relieve enhancement of electric field at the corner portion of trench filled with insulator.

[0003] 2. Description of the Related Art

[0004] Generally, an isolation region is needed to make, first of all, in order to insulate each element, when plural elements are going to be formed on a semiconductor substrate. This kind of isolation region is made by method, for example, shown in FIG. 6.

[0005] At first, as shown in FIG. 6(a), a mask for forming trench is formed. This mask is formed by etching a silicon nitride film 3 and a silicon oxide film 2 in this order, with photo-lithography technique, after forming a silicon oxide film 2 and a silicon nitride film 3 in this order.

[0006] And, as shown in FIG. 6(b), a trench 4 is formed by etching substrate 1 with this mask.

[0007] After this, insulating material is going to be filled in this trench 4. However, before that, the corner portion A of trench 4 needs to be formed to a gentle R-shape. The reason is described briefly, as follows.

[0008] As shown in FIG. 3., plural elements, that is, MOS transistors 10,10 cross their common gate electrode 11 with these corner portions A. And, the electric field impressed to the gate electrode 11 is provided to an active region between source region 12 and drain region 13 (c.f. FIG. 4).

[0009] However, if these corner portions A are left sharp, then, the electric field impressed to the gate electrode 11 enhances at these sharp corner portions. Therefore, dielectric break down is likely to be caused by the concentration of electric field at corner portions A. And, the thinner becomes the gate insulating film, the more increases the necessity of relieving this electric field concentration. Moreover, parasitic transistors should have been caused by the concentration of electric field or thinning of gate insulating film, as well as drop of reliability of gate insulating film is caused.

[0010] Therefore, as shown in FIG. 6(c), the trench corner portions A are formed to R-shape by forming thermal oxide film 5 with heat-oxidation. After this, as shown in FIG. 6(d), a silicon oxide film 6 is filled in the trench 4. And, the silicon nitride film 3 and the silicon oxide film 2 are removed. Then, a gate oxide film 7 is formed. Meantime, to make gentle these trench corner portions A effectively, a method of forming shallow and wide trench is adopted (c.f. JP 2000-306991).

[0011] Moreover, as another conventional art, there is a method of supplying silicon oxide film with fluorine only in the vicinity of trench corner portions A. This method is processed by implanting fluorine ion etc. in the trench corner portions A mentioned above, with Tilt Ion Implantation technique (c.f. JP 10-270546 especially in page 4, FIG. 1).

[0012] Besides, there is another method of filling the trench with fluoride SiOF as well as forming silicon oxide film at trench corner portions A (c.f. JP 11-186378 especially in pages 7,8, FIGS. 1 to 5).

[0013] However, the conventional methods mentioned above had next problems.

[0014] First, as for the method of forming thermal oxide film, it had various disadvantages owing to the high temperature by thermal oxidation. To be concrete, high temperature of 1000° C. to 1200° C. was needful. And, film thickness of 30 nm to 60 nm was needful in order to make corner portions gentle effectively. So, the thermal process of high temperature mentioned above, must be continued, while the film of this thickness is formed. Moreover, high ability of oxidation was needful as well as high temperature, because the rate of oxidation must be low. That is, wet oxidation is more preferable than dry oxidation. Further, radical oxidation etc. is still more preferable.

[0015] Therefore, in manufacturing process, pollution in a chamber of furnace, tube or boat is likely to occur. And, slips or warp of wafer is likely to occur.

[0016] Moreover, an expensive and specific apparatus was necessary, because a function for retarding oxidation rate at high temperature and a function for performing radical oxidation are needful. Therefore, cost up of product was inevitable.

[0017] Second, there is a problem caused by high integration of elements. That is, the thickness of thermal oxidation film 5 must be made thinner, as the semiconductor elements become thinner, in the occasion of using thermal oxidation film 5 for gentling corner portions A. Otherwise, the transforming difference, which occur in the occasion of transforming silicon into silicon oxide with thermal oxidation, becomes large, compared with the size of the elements. However, an oxide film having thickness of more than 30 nm mentioned above is needful in order to make the corner portions sufficiently gentle. Therefore, it has become difficult to make it gentle.

[0018] Third, damage of substrate or pollution of metal can be caused in the occasion of using an ion implantation technique. In order to avoid this, it is necessary to make the film thickness of mask sufficiently thick when it is used for ion implantation. To be concrete, thickness of more than 180 nm is needful at 30 keV of ion implantation acceleration.

[0019] Fourth, as for the method of filling fluoride all over inside of trench, it is difficult to fill trench inside uniformly. So, cavities are likely to occur inside trench. Moreover, a lot of fluorine is included in other portions than the trench corner portions A where electric field concentrates. Such a lot of fluorine is likely to invade into active regions by heat-diffusion, when a gate oxide film etc. is formed. By this, characteristic of elements is likely to deteriorate.

SUMMARY OF THE INVENTION

[0020] The present invention adopts next configuration in order to solve the problems mentioned above.

[0021] As a first configuration, at first, performed are a process of forming a silicon oxide film on a substrate, a process of forming a silicon nitride film on the silicon oxide film, and a process of forming a trench opening by patterning these silicon nitride film and silicon oxide film; so as to form an oxide film with fluorine on the substrate where the trench opening is formed.

[0022] And, performed are a process of forming a fluorine diffusion layer by diffusing fluorine with heat-diffusion from the oxide film with fluorine, and a process of removing the oxide film with fluorine; so as to form a trench by using these silicon nitride film and silicon oxide film as a mask.

[0023] After these processes, performed are a process of forming thermal oxide film which includes fluorine atoms at corner portions, in a process of forming thermal oxide film on inner side walls of the trench, and filling oxide film in the trench, and a process of removing said silicon nitride film and silicon oxide film; so as to make isolation regions of semiconductor elements.

[0024] As a second configuration, at first, performed are a process of forming a silicon oxide film on a substrate, a process of forming a silicon nitride film on the Silicon oxide film, and a process of forming a trench opening by patterning these silicon nitride film and silicon oxide film; so as to form oxide film including fluoride on the substrate where the trench opening is formed, and form side walls by etching the oxide film with fluorine.

[0025] And, performed is a process of forming a trench by using these side walls, said silicon nitride film and silicon oxide film as a mask.

[0026] After this process, performed are a process of forming thermal oxide film including fluorine atoms at corner portions, in a process of forming thermal oxide film on the inner wall of the trench, and filling the trench with oxide film, and a process of removing said silicon nitride film and silicon oxide film; so as to make isolation region of semiconductor element.

[0027] Since, the present invention adopts the method of forming oxide film including fluorine atoms only at the trench corner portions, the oxide film of trench corner portions can be made electrically thick without using expensive manufacturing apparatus. By this, concentration of electric field at trench corner portions can be relieved. Besides, occurrence of surface state and occurrence of positive hole trapping can be suppressed. Moreover, thermal oxide film formed on the inner side walls can be made thinner corresponding to thinner film of semiconductor elements. So, the difference of the position of trench inner wall surface caused by the transformation from silicon to silicon oxide with heat-oxidation, can be suppressed. By this, progression of oxidation into active regions where minute elements are formed, is suppressed. As a result, reliability of gate insulating film can be increased as well as yield of semiconductor elements can be kept in high level. So, device of high quality can be provided.

[0028] On the other hand, including fluorine atoms vainly all over in the trench is avoided, because they can be included only at the trench corner portions A. By this, the threat of diffusing fluorine impurities into the transistor forming region, has vanished. Thus, fluorine atoms can be included effectively only in the regions where electric field concentration occurs.

BRIEF DESCRIPTION OF THE DRAWINGS

[0029]FIG. 1 shows Embodiment 1 of manufacturing method according to present invention.

[0030]FIG. 2 shows the trench corner portion featuring present invention.

[0031]FIG. 3 shows a bird view of elements and their isolating region.

[0032]FIG. 4 shows configuration of an element.

[0033]FIG. 5 shows Embodiment 2 of manufacturing method according to present invention.

[0034]FIG. 6 shows an example of conventional manufacturing method.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0035] Hereafter, preferred embodiments of present invention are described referring to the drawings.

[0036] <Embodiment 1>

[0037]FIG. 1 shows principal manufacturing processes in a method for manufacturing isolating region of semiconductor element, especially isolating layer with trench.

[0038] In FIG. 1(a), at first, on a substrate 1, a silicon oxide film 2 is formed at 5 to 50 nm as a basement layer by CVD (Chemical Vapor Deposition) or heat-oxidation. And, thereon, a silicon nitride layer 3 is formed by CVD. After this, a patterning is performed to this silicon nitride film 3 and the silicon oxide film 2 beneath it, by photo-lithography and etching.

[0039] And, on the substrate patterned like this, an oxide film including fluorine comprising SiOF is formed at 50 to 200 nm by plasma CVD. This can be performed, for example, by using high density plasma, in an atmosphere of mixture gas of SiH4, SiF4, O2 and Ar; at a power of about 4 kW and a pressure of about 800 pa.

[0040] Next to this, an anneal is performed for 10 to 30 minutes at a temperature of 900 to 1000° C. By this, fluorine atoms are heat-diffused into region of substrate 1 where elements isolating region is formed, with the patterned silicon nitride film 3 as a diffusion mask. As a result, a fluorine diffusing layer 9 is formed. Incidentally, in the process, the active regions of substrate 1 where elements are formed in the future, are protected by the patterned silicon nitride film 3, which covers the active regions. So, the active regions are never polluted.

[0041] In FIG. 1(b), at first, the oxide film including fluorine 8 is removed with a solution of fluoric acid. And, a trench 4 is formed in the elements isolating region of substrate 1 by etching known in public, with using silicon oxide film 2 and silicon nitride film 3 as a mask. In this occasion, almost all of fluorine diffusing region 9 is removed by etching. However, small parts of it are remained as silicon layer portions containing fluorine 9 a in the vicinity of end portions of the patterned silicon oxide film 2.

[0042] In FIG. 1(c), a thermal oxide film 5 of thickness of 5 to 15 nm is formed by performing heat-oxidation at temperature of 900 to 1100° C. This is performed for gentling the corner portions A of trench 4. The thermal oxide film 5 formed in this way becomes oxide layer portion including fluorine 5 a at the portion in the vicinity of silicon layer portion including fluorine 9 a. Incidentally, in the occasion of this heat-oxidation, active regions of substrate 1 are protected from pollution by the silicon nitride film 3 formed sufficiently thick.

[0043] In FIG. 1(d), an oxide film 6 is formed by CVD. And, it is filled in by CMP (Chemical Mechanical Polishing). After this, the silicon nitride film 3 and silicon oxide film 2 are removed.

[0044] After that, a gate oxide film 7 is formed by heat-oxidation. The gate oxide film 3 formed in this way becomes an oxide layer portion including fluorine 7 a in the vicinity of the silicon layer portion including fluorine 9 a, by diffusing fluorine atoms with heat-oxidation. Moreover, the gate oxide film 7 can be formed by CVD. The gate oxide film 7 formed in this way becomes an oxide layer portion including fluorine 7 a by heat-diffusion of fluorine atoms with annealing process.

[0045] As mentioned above, a silicon oxide film including fluorine only in the vicinity of trench corner portions A.

[0046]FIG. 3 is a bird view of substrate shown in FIG. 1(d), and FIG. 4 is a sectional view of line B-B shown in FIG. 3.

[0047] As mentioned above, after forming an isolating region of elements, as shown in FIG. 4, a gate electrode 11 is formed on the gate oxide film 7. And, succeedingly, an impurity is introduced with using the gate electrode 11 as a mask. Then, a source region 12 and a drain region 13 are formed. Thus, a MOS transistor 10 is formed.

[0048] <Function of Embodiment 1>

[0049] Fluorine atoms included in silicon have next function. The specific inductive capacity of silicon oxide film is usually about 3.9. However, it becomes about 3.7 if fluorine atoms are included at 5 atm % for example. And, it becomes about 3.5 if fluorine atoms are included at 10 atm % for example. In the occasion when the specific inductive capacity reduced in this way, the electric field is relived at the portions where it reduced. Therefore, the concentration of electric field is avoided. This is, so to speak, the thickness of film is electrically made thick at the portions including fluorine atoms, though the material thickness is same. That is, the electrical thickness is thicker than the material thickness by about 5% if 5 atm % of fluorine atoms are included. And, it is about 10% if 10 atm % of them are included. Moreover, deterioration of quality of films in the occasion when fluorine atoms are introduced, does not occur.

[0050] Thus, deterioration of characteristic of semiconductor element caused by the concentration of electric field with abrupt increase of curvature at trench corner portions A or the concentration of electric field with progressive thinning of film, is relived. And, causing parasitic transistors is avoided.

[0051] Moreover, mechanical stress is likely to concentrate upon trench corner portions A. Then, surface state occurs thereon. So, positive holes are likely to be trapped thereon. As a result, for example, power of element decreases. And, characteristic of device is badly influenced by this. It has been understood that fluorine atoms included at trench corner portions A suppress this mechanical stress.

[0052] <Effect of Embodiment 1>

[0053] As mentioned above, according to Embodiment 1, the oxide film at trench corner portions A can be made thicker electrically without using an expensive manufacturing apparatus; because SiOF film is formed after patterning a masking material for forming trench, fluorine atoms are diffused from this SiOF film only into trench corner portions A, and an oxide film including fluorine atoms only at the trench corner portions A is formed by heat-oxidation performed after that. By this effect, concentration of electric field at trench corner portions A can be depressed. Besides, causing surface state or trapping of positive holes can be suppressed. Moreover, thermal oxide film formed on the inner side walls of trench, can be made thinner corresponding to thinning of elements. And, increase of the difference of interface position caused by the transformation from silicon to silicon oxide with heat-oxidation, can be suppressed. By this effect, progression of oxidation into active regions where miniaturized elements are formed, is suppressed. As a result, a high yield can be conserved. Besides, reliability of gate insulating film can be enhanced. Thus, device of high quality can be provided.

[0054] Moreover, according to Embodiment 1, ion implantation is never used in the occasion when fluorine atoms are introduced. So, damage to substrate and pollution of metal does not occur. Meanwhile, it is necessary to prevent fluorine ions from invading into regions where transistors are formed, not to deteriorate characteristic of transistors. So, it is necessary to make thickness of mask sufficiently thick. However, it is sufficient as 10 nm.

[0055] On the other hand, according to Embodiment 1, including fluorine atoms vainly all over in trench, is avoided, because fluorine atoms can be included only at the trench corner portions A. By this effect, a threat of diffusing fluorine impurity into transistor forming region, is vanished. Thus, the merit of present invention is that fluorine atoms are included effectively only at portions where electric field concentration occurs.

[0056] <Embodiment 2>

[0057]FIG. 5 shows principal manufacturing processes in a method of manufacturing semiconductor device according to present invention.

[0058] In FIG. 5(a), at first, a silicon oxide film 2 is formed at 5 to 50 nm on a substrate 1 as a basement by CVD or heat-oxidation. And, thereon, a silicon nitride film 3 is formed by CVD. After that, patterning is performed to this silicon nitride film 3 and the silicon oxide film beneath it, by photo-lithography and etching.

[0059] And, on the substrate 1 patterned like this, an oxide film including fluorine 8 comprising SiOF is formed at 50 to 200 nm by plasma CVD. The process mentioned above are performed as same as Embodiment 1.

[0060] In FIG. 5(b), succeeding to the processes mentioned above, some portions of the oxide film including fluorine 8 are removed, so as to form side walls 8 a. This is performed by an-isotropic etching known in public, removing them by the etching only in the vertical direction in the drawing. Then, a trench opening is formed between these side walls 8 a. Therefore, as for the opening formed by patterning to the silicon nitride film 3 and silicon oxide film 2, its width is the trench opening's width plus both widths of the side walls 8 a. So, the patterning mentioned above is performed at this width. Moreover, the trench opening's width is adjusted in detail by adjusting the thickness of the oxide film including fluorine 8 in a range of 50 to 200 nm.

[0061] In FIG. 5(c), a trench 4 is formed in an isolating region for element of substrate 1 by etching known in public; with using the silicon oxide film 2, silicon nitride film 3 and side walls 8 a as a mask.

[0062] In FIG. 5(d), a thermal oxide film 5 is formed at the thickness of 5 to 15 nm by performing heat-oxidation at the temperature of 900 to 1100° C. This is what is performed for gentling corner portions A of trench 4. In the occasion of this heat-oxidation, at the portions of substrate 1 which face the side walls 8 a, fluorine atoms diffuse from side walls 8 a by heat-diffusion. Then, the more fluorine is included in the more upper portions of thermal oxide film 5. Besides, fluorine containing silicon layer portions 9 a are formed in the vicinity of the side walls 8 a of the substrate 1.

[0063] In FIG. 5(e), an oxide film 6 is formed by CVD. Then, it is filled by CMP. And, the silicon nitride film 3 and silicon oxide film 2 are removed.

[0064] After that, gate oxide film 7 is formed by heat-oxidation or CVD. The gate oxide film 7 formed in this way becomes fluorine containing oxide layer portions 7 a in the vicinity of fluorine containing silicon layer portions 9 a, by heat-diffusion with heat-oxidation or annealing.

[0065] As mentioned above, a silicon oxide film including only in the vicinity of trench corner portions A, is formed.

[0066] This is almost same as the silicon oxide film formed by the manufacturing method of Embodiment 1. And, the function of it is same as the function described in Embodiment 1.

[0067] Moreover, as mentioned above, after forming an isolating region of elements, as shown in FIG. 4, a gate electrode 11 is formed on the gate oxide film 7. And, succeedingly, an impurity is introduced with using the gate electrode 11 as a mask. Then, a source region 12 and a drain region 13 are formed. Thus, a MOS transistor 10 is formed.

[0068] <Effect of Embodiment 2>

[0069] As mentioned above, according to Embodiment 2, a fluorine diffusing layer can be easily obtained without forming a fluorine diffusing layer 9 being different from Embodiment 1, because side walls 8 a comprising SiOF film is formed after patterning a masking material for forming trench, fluorine atoms are diffused from this SiOF film only into trench corner portions A, and an oxide film including fluorine atoms only at the trench corner portions A is formed by heat-oxidation performed after that. Moreover, fluorine atoms can be easily diffused, even if width of trench opening becomes narrow as high integration and thinning of film proceed. As a result, the oxide film at trench corner portions A can be made thicker electrically without using an expensive manufacturing apparatus. By this effect, concentration of electric field at trench corner portions A can be depressed. Besides, causing surface state or trapping of positive holes can be suppressed. Moreover, thermal oxide film formed on the inner side walls of trench, can be made thinner corresponding to thinning of elements. And, increase of the difference of interface position caused by the transformation from silicon to silicon oxide with heat-oxidation, can be suppressed. By this effect, progression of oxidation into active regions where miniaturized elements are formed, is suppressed. As a result, a high yield can be conserved. Besides, reliability of gate insulating film can be enhanced. Thus, device of high quality can be provided.

[0070] Moreover, according to Embodiment 2 similarly as Embodiment 1, ion implantation is never used in the occasion when fluorine atoms are introduced. So, damage to substrate and pollution of metal do not occur. And, the thickness of mask to prevent fluorine ions from invading into regions where transistors are formed, is sufficient as 10 nm.

[0071] On the other hand, according to Embodiment 2, similarly as Embodiment 1, including fluorine atoms vainly all over in trench, is avoided, because fluorine atoms can be included only at the trench corner portions A. By this effect, a threat of diffusing fluorine impurity into transistor forming region, is vanished. Thus, in Embodiment 2 similarly as Embodiment 1, the merit is that fluorine atoms are included effectively only at portions where electric field concentration occurs.

[0072] <Other Embodiment>

[0073] In each Embodiment mentioned above, an oxide film including fluorine atoms, is formed as SiOF film by CVD. And, this film is used as a source of diffusing fluorine atoms. However, the present invention is not limited to this. A non-doped oxide film not including fluorine atoms, can be formed by CVD. And, after that, fluorine atoms can be introduced into this CVD film by ion implantation. 

What is claimed is:
 1. A method of manufacturing semiconductor device comprising a process of preparing a semiconductor substrate having at least one region to form isolating layer for element on the main surface of the substrate, a process of forming an oxide film including fluorine on said main surface of said region to form isolating layer for element, a process of forming fluorine atoms diffusing region on said main surface corresponding to the peripheral portion of said region to form isolating layer for element, by diffusing fluorine atoms from said oxide film including fluorine to said main surface, a process of forming a trench on said main surface corresponding to said region to form isolating layer for element, in order to remain said fluorine atoms diffusing region at peripheral portion of the trench, and a process of forming isolating layer for element, by filling said trench selectively with oxide film.
 2. A method of manufacturing semiconductor device according to claim 1 wherein the forming condition of said fluorine atoms diffusing region is decided that the width of said fluorine atoms diffusing region is broader than the width of said isolating layer for element.
 3. A method of manufacturing semiconductor device according to claim 1 wherein the size of the remaining portion of said fluorine atoms diffusing region is decided to relieve the concentration of electric field at the end of said isolating layer for element.
 4. A method of manufacturing semiconductor device comprising a process of forming silicon oxide film on substrate, a process of forming silicon nitride film on the silicon oxide film, a process of forming trench opening by patterning the silicon nitride film and the silicon oxide film, a process of forming oxide film including fluorine on the substrate where the trench opening is formed, a process of forming fluorine diffusing layer by diffusing fluorine from the oxide film including fluorine with heat-diffusion, a process of removing said oxide film including fluorine, a process of forming trench with using said silicon nitride film and silicon oxide film as mask, a process of forming thermal oxide film on the inner wall of trench, a process of filling the trench with oxide film, and a process of forming isolating region for element by removing said silicon nitride film and silicon oxide film.
 5. A method of manufacturing semiconductor device according to claim 4, wherein the processes added after said process of forming isolating region for element are comprising a process of forming gate oxide film with heat-oxidation, a process of forming gate electrode, and a process of forming source region and drain region.
 6. A method of manufacturing semiconductor device according to claim 4, wherein the processes added after said process of forming isolating region for element are comprising a process of forming gate oxide film with Chemical Vapor Deposition, a process of annealing the gate oxide film, a process of forming gate electrode, and a process of forming source region and drain region.
 7. A method of manufacturing semiconductor device according to claim 4, wherein said thermal oxide film formed on the inner wall of trench is not more than 30 nm.
 8. A method of manufacturing semiconductor device comprising a process of forming silicon oxide film on substrate, a process of forming silicon nitride film on the silicon oxide film, a process of forming trench opening by patterning the silicon nitride film and the silicon oxide film, a process of forming oxide film including fluorine on the substrate where the trench opening is formed, a process of forming side wall by etching the oxide film including fluorine, a process of forming trench with using said side wall, silicon nitride film and silicon oxide film as mask, a process of forming thermal oxide film on the inner wall of trench, a process of filling the trench with oxide film, and a process of forming isolating region for element by removing said silicon nitride film and silicon oxide film.
 9. A method of manufacturing semiconductor device according to claim 8, wherein the processes added after said process of forming isolating region for element are comprising a process of forming gate oxide film with heat-oxidation, a process of forming gate electrode, and a process of forming source region and drain region.
 10. A method of manufacturing semiconductor device according to claim 8, wherein the processes added after said process of forming isolating region for element are comprising a process of forming gate oxide film with Chemical Vapor Deposition, a process of annealing the gate oxide film, a process of forming gate electrode, and a process of forming source region and drain region.
 11. A method of manufacturing semiconductor device according to claim 8, wherein said thermal oxide film formed on the inner wall of trench is not more than 30 nm. 